1. Field of the Invention
This invention relates generally to a disc brake for automotive vehicles and more particularly to a disc brake of the pin-guide type in which the sliding movement of a caliper is guided by guide pins.
2. Prior Art
In a pin-guide type disc brake, a caliper is mounted by a pair of guide pins on a mounting bracket for sliding movement along the guide pins relative to the mounting bracket, and the sliding portions are completely covered by elastic protective boots. Therefore, the pin-guide type disc brake is advantageous over a disc brake of the caliper-floating type in that the sliding portions are less affected by foreign matters such as muddy water and dirt.
Generally, each of the guide pins is slidably extended through a mounting arm of the caliper, and a pair of protective boots sealingly covers the opposite end portions of the guide pin extending from the mounting arm, with one end of the boots being engaged respectively in peripheral mounting grooves formed in the guide pin. This technique is disclosed in Japanese Utility Model Application Laid-Open (Kokai) Nos. 56-71536 and 55-63436 and Japanese Patent Application Laid-Open (Kokai) No. 57-157833.
The provision of the mounting grooves in the guide pin has lowered the strength thereof and made the shape complicated. In addition, the seal between the boot and the guide pin has not been satisfactory, so that foreign matters such as dirt tend to intrude into the sliding portions of the caliper. Particularly, as the caliper is slidingly moved along the guide pins, one of the pair of boots is compressed while the other is evacuated, and this produces a pumping action, so that dirt or the like tends to be drawn into the boots. In addition, such an action increases resistance to the movement of the caliper. For this reason, it is important to provide a positive seal between the boot and the guide pin. One solution to the problem of the pumping action has been proposed in Japanese Utility Model Publication (Kokai) No. 58-32026 in which a slit is formed in the sealing portion of each of the two boots so that the interiors of the two boots are communicated with each other through a gap between the area of sliding contact between the caliper and the guide pins. Generally, oily material such as grease is applied to such sliding contact area to facilitate the sliding movement, and such grease tends to close the slits to adversely affect the communication between the interiors of the two boots.